My vet suggests alternating by years. Cows and calves get poured during spring and fall herd workups. One year we use Ivermectin for both pours, the next year Cydectin. He uses the real thing.  Generics are sold on price. Maybe they aren't exactly the same?

For as little as you use twice a year and the potential problems if they don't work, maybe this is not a place for generics?

So far I have had no sign of internal parasites.  A couple years ago I did have some late winter mange, probably from a coyote, but one shot of injectible Ivermectin took care of that.

A key thing on all of them is to get the dosage right and for that you need the weight of the animal. Mine get poured as they come off the scale and the vet adjusts the dose based on each animal's measured weight, not a guess.

Jim